1. Field of the Invention
The present invention relates to computer systems and to user input devices for them.
2. Description of the Related Art
The explosion of the personal computer market has lead to great advances and ease of use of those personal computers. The keyboard and text display has given way to a variety of user input devices for control of graphical interfaces. These input devices either augment or totally replace the keyboard, and include mice, track balls, digitizer pads, touch sensitive screens, among other things.
So far as is known, there have been three principal technologies used for digitizer pads and touch sensitive screens. These are known as capacitive technology, resistive technology, and surface acoustic wave (SAW) technology.
Resistive technology typically used a voltage gradient on a plastic on glass membrane overlay to sense touch. For example, a five wire sense system created a voltage gradient on the bottom layer, and the top layer senses that voltage. The two layers of the screens were coated with a thin, clear conductive metal oxide on their facing sides and held apart by a layer of materials composed of spacer dots. Along the edges of the sensor was electrode pattern. The controller dispersed a uniform voltage field across the sensor and then measured the voltage on the glass layer at the location where a user's finger or other indicator object pressed the two layers together. The sensed voltage was then translated into a set of digital touch coordinates by a controller and sent to a host computer.
Capacitive digitizer technology typically used an all glass sensor with a transparent, thin film conductive coating fused to its surface. Along the edges was a narrow, precisely printed electrode pattern that uniformly distributed low voltage, AC field over the conduct layer. When a finger made contact with the screen surface, it "capacitively coupled" with the voltage field, drawing a minute amount of current to the point of contact. The current flow from each corner was proportional to the distance to the finger and the ratios of these flows were measured by the controller and used to locate the touch.
Surface acoustic wave (SAW) technology provided a glass panel with transducers that transmitted and received surface waves over the face of the screen. When a finger or other object touched the screen, a portion of the energy of the wave was absorbed at the touch location. This location could then be determined based on the presence of interference patterns caused in the acoustic wave.